Incorrect behavior of hint in multiline TextBox (continuation)

[omllmo]

Bug explanation

Сode:

<Window x:Class="WpfApp1.MainWindow"
        xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
        xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
        xmlns:d="http://schemas.microsoft.com/expression/blend/2008"
        xmlns:mc="http://schemas.openxmlformats.org/markup-compatibility/2006"
        xmlns:materialDesign="http://materialdesigninxaml.net/winfx/xaml/themes"
        xmlns:local="clr-namespace:WpfApp1"
        mc:Ignorable="d"
        Title="MainWindow" Height="450" Width="800"
        Loaded="Window_Loaded">
    <materialDesign:DialogHost Name="dialogHost" Margin="10 0 0 0"
                               Style="{StaticResource MaterialDesignEmbeddedDialogHost}">
        <materialDesign:DialogHost.DialogContent>
            <Grid>
                <Grid Name="gridInDialogHost" Width="300"
                      Margin="15"
                      DataContext="{x:Null}" >
                    <Grid.RowDefinitions>
                        <RowDefinition Height="Auto"/>
                        <RowDefinition Height="Auto"/>
                        <RowDefinition Height="Auto"/>
                    </Grid.RowDefinitions>
                    <materialDesign:PackIcon Kind="PencilBoxMultiple" Height="25" Width="25"/>
                    <TextBlock Text="Test"
                               Margin="30 2 0 10" Grid.ColumnSpan="3"
                               VerticalAlignment="Bottom"
                               FontSize="15"/>
                    <Border HorizontalAlignment="Stretch" VerticalAlignment="Bottom"
                            BorderBrush="Blue" BorderThickness="1" Height="1"/>
                    <TextBox Grid.Row="1" Margin="10 20 0 10"
                             Text="{Binding Comment, Mode=TwoWay, UpdateSourceTrigger=PropertyChanged}"
                             VerticalAlignment="Stretch"
                             HorizontalAlignment="Stretch"
                             materialDesign:HintAssist.Hint="HINT TEST"
                             materialDesign:HintAssist.FloatingScale="1"
                             materialDesign:TextFieldAssist.HasClearButton="True"
                             materialDesign:TextFieldAssist.IconVerticalAlignment="Top"
                             AcceptsReturn="True"
                             MaxLength="300"
                             Style="{StaticResource MaterialDesignFilledTextBox}"
                             TextWrapping="Wrap"
                             FontSize="14"/>
                    <Button Content="Close"
                            Grid.Row="2" Margin="0 25 5 5" HorizontalAlignment="Right" 
                            Width="100"
                            Command="{x:Static materialDesign:DialogHost.CloseDialogCommand}"
                            IsCancel="True"/>
                </Grid>
            </Grid>
        </materialDesign:DialogHost.DialogContent>
        <DataGrid Name="dataGrid"
                  FontSize="14" 
                  MinHeight="150"
                  AutoGenerateColumns="False" 
                  IsReadOnly="True" 
                  SelectionMode="Single"
                  MouseDoubleClick="DG_MouseDoubleClick">
            <DataGrid.Columns>
                <DataGridTextColumn Header="Description" 
                                    Binding="{Binding Comment}" 
                                    Foreground="Red"/>
            </DataGrid.Columns>
        </DataGrid>
    </materialDesign:DialogHost>
</Window>

---

    public partial class MainWindow : Window
    {
        ObservableCollection<TestClass> list;
        TestClass testClass;

        public MainWindow()
        {
            InitializeComponent();
        }

        private void Window_Loaded(object sender, RoutedEventArgs e)
        {
            list = new ObservableCollection<TestClass>
            {
                new TestClass() { Comment = "test test test test test test test test test test test test  test test test test test test test test test test test " }
            };

            dataGrid.ItemsSource = list;
        }

        private void DG_MouseDoubleClick(object sender, MouseButtonEventArgs e)
        {
            if (e.LeftButton == MouseButtonState.Pressed)
            {
                if (dataGrid.SelectedItem is TestClass tc && DataGrid.GetIsSelectionActive(VisualTreeHelper.HitTest(dataGrid, e.GetPosition(dataGrid)).VisualHit))
                {
                    gridInDialogHost.DataContext = null;

                    testClass = new TestClass() { Comment = tc.Comment };

                    gridInDialogHost.DataContext = testClass;

                    dialogHost.IsOpen = true;
                }
            }
        }
    }

---

public class TestClass : INotifyPropertyChanged
    {
        public event PropertyChangedEventHandler PropertyChanged;

        private int id;
        public int ID { get => id; set { id = value; RaisePropertyChanged("ID"); } }

        private string comment;
        public string Comment { get => comment; set { comment = value; RaisePropertyChanged("Comment"); } }

       
        protected virtual void RaisePropertyChanged(PropertyChangedEventArgs e)
        {
            PropertyChanged?.Invoke(this, e);
        }
        protected void RaisePropertyChanged(string propertyName)
        {
            RaisePropertyChanged(new PropertyChangedEventArgs(propertyName));
        }
    }

Version

5.1.1-ci780